Semiconductor device having cap layer

ABSTRACT

A semiconductor device includes a semiconductive substrate, a semiconductive fin, an isolation structure, a source/drain epitaxial structure, a first cap layer, and a second cap layer. The semiconductive fin protrudes from the semiconductive substrate. The isolation structure is over the semiconductive substrate and laterally surrounds the semiconductive fin. The source/drain epitaxial structure is over the semiconductive fin. The source/drain epitaxial structure has a rounded corner extending laterally and a top above the rounded corner. The first cap layer extends from the rounded corner of the source/drain epitaxial structure to the top of the source/drain epitaxial structure. The second cap layer covers the rounded corner and a bottom of the source/drain epitaxial structure. The first and second cap layers are made of different materials.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.16/927,799, filed Jul. 13, 2020, which a continuation of U.S. patentapplication Ser. No. 16/178,340, filed Nov. 1, 2018, now U.S. Pat. No.10,714,475, issued Jul. 14, 2020, which claims priority to U.S.Provisional Application Ser. No. 62/591,133, filed Nov. 27, 2017, bothof which are herein incorporated by reference in their entirety.

BACKGROUND

Transistors include semiconductor regions used to form the sourceregions and drain regions. Since the contact resistance between metalcontact plugs and the semiconductor regions is high, metal silicides areformed on the surfaces of the semiconductor regions such as siliconregions, germanium regions, silicon germanium regions in order to reducethe contact resistance. The contact plugs are formed to contact thesilicide regions, and the contact resistance between the contact plugsand the silicide regions are low.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1-15 illustrate a method for manufacturing a semiconductor deviceat various stages in accordance with some embodiments of the presentdisclosure.

FIGS. 16-22 illustrate a method for manufacturing a semiconductor deviceat various stages in accordance with some embodiments of the presentdisclosure.

FIG. 23 is a cross-sectional view of a semiconductor device inaccordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The fins may be patterned by any suitable method. For example, the finsmay be patterned using one or more photolithography processes, includingdouble-patterning or multi-patterning processes. Generally,double-patterning or multi-patterning processes combine photolithographyand self-aligned processes, allowing patterns to be created that have,for example, pitches smaller than what is otherwise obtainable using asingle, direct photolithography process. For example, in one embodiment,a sacrificial layer is formed over a substrate and patterned using aphotolithography process. Spacers are formed alongside the patternedsacrificial layer using a self-aligned process. The sacrificial layer isthen removed, and the remaining spacers may then be used to pattern thefins.

Embodiments of the present disclosure provide some improved methods forthe formation of semiconductor devices and the resulting structures.These embodiments are discussed below in the context of forming finFETtransistors having a single fin or multiple fins on a bulk siliconsubstrate.

FIGS. 1-15 illustrate a method for manufacturing a semiconductor deviceat various stages in accordance with some embodiments of the presentdisclosure. In some embodiments, the semiconductor device shown in FIGS.1-15 may be intermediate devices fabricated during processing of anintegrated circuit (IC), or a portion thereof, that may include staticrandom access memory (SRAM), logic circuits, passive components, such asresistors, capacitors, and inductors, and/or active components, such asp-type field effect transistors (PFETs), n-type FETs (NFETs), multi-gateFETs, metal-oxide semiconductor field effect transistors (MOSFETs),complementary metal-oxide semiconductor (CMOS) transistors, bipolartransistors, high voltage transistors, high frequency transistors, othermemory cells, and combinations thereof.

FIG. 1 is a perspective view of the semiconductor device in accordancewith some embodiments of the present disclosure. A substrate 110 isprovided. The substrate 110 has a first region 102 and a second region104. In some embodiments, the substrate 110 may include silicon (Si).Alternatively, the substrate 110 may include germanium (Ge), silicongermanium, gallium arsenide (GaAs), or other appropriate semiconductormaterials. In some alternative embodiments, the substrate 110 mayinclude an epitaxial layer. Furthermore, the substrate 110 may include asemiconductor-on-insulator (SOI) structure having a buried dielectriclayer therein. The buried dielectric layer may be, for example, a buriedoxide (BOX) layer. The SOI structure may be formed by a method referredto as separation by implantation of oxygen (SIMOX) technology, waferbonding, selective epitaxial growth (SEG), or other appropriate method.

A plurality of semiconductor fins 112 and a plurality of semiconductorfins 114 are respectively formed over the first region 102 and thesecond region 104 of the substrate 110. The semiconductor fins 112 and114 serve as channels and source/drain features of transistors. It isnoted that the numbers of the semiconductor fins 112 and 114 in FIG. 1are illustrative, and should not limit the claimed scope of the presentdisclosure. In addition, one or more dummy fins may be disposed adjacentboth sides of the semiconductor fins 112 and/or the semiconductor fins114 to improve pattern fidelity in patterning processes.

The semiconductor fins 112 and 114 may be formed, for example, bypatterning and etching the substrate 110 using photolithographytechniques. In some embodiments, a layer of photoresist material (notshown) is deposited over the substrate 110. The layer of photoresistmaterial is irradiated (exposed) in accordance with a desired pattern(the semiconductor fins 112 and 114 in this case) and developed toremove a portion of the photoresist material. The remaining photoresistmaterial protects the underlying material from subsequent processingoperations, such as etching. It should be noted that other masks, suchas an oxide or silicon nitride mask, may also be used in the etchingprocess. The semiconductor fins 112 and 114 may be made of the samematerial as the substrate 110 and may continuously extend or protrudefrom the substrate 110. The semiconductor fins 112 and 114 may beintrinsic, or appropriately doped with an n-type impurity or a p-typeimpurity.

In some other embodiments, the semiconductor fins 112 and 114 may beepitaxially grown. For example, exposed portions of an underlyingmaterial, such as an exposed portion of the substrate 110, may be usedin an epitaxial process to form the semiconductor fins 112 and 114. Amask may be used to control the shape of the semiconductor fins 112 and114 during the epitaxial growth process.

A plurality of isolation structures 120, such as shallow trenchisolation (STI), are formed in the substrate 110 to separate variousdevices. The formation of the isolation structures 120 may includeetching a trench in the substrate 110 and filling the trench by aninsulator material such as silicon oxide, silicon nitride, or siliconoxynitride. The filled trench may have a multi-layer structure such as athermal oxide liner layer with silicon nitride filling the trench. Insome embodiments, the isolation structures 120 may be created using aprocess sequence such as: growing a pad oxide, forming a low pressurechemical vapor deposition (LPCVD) nitride layer, patterning an STIopening using photoresist and masking, etching a trench in the substrate110 (to form the semiconductor fins 112 and 114), optionally growing athermal oxide trench liner to improve the trench interface, filling thetrench with oxide, using chemical mechanical planarization (CMP) toremove the excessive oxide, and recessing the thermal oxide trench linerand the oxide to form the isolation structures 120 such that topportions of the semiconductor fins 112 and 114 protrude from topsurfaces of the isolation structures 120.

Reference is made to FIG. 2 . A dummy dielectric layer 210 isconformally formed to cover the semiconductor fins 112, 114, and theisolation structures 120. In some embodiments, the dummy dielectriclayer 210 may include silicon dioxide, silicon nitride, a high-κdielectric material, or other suitable material. In various examples,the dummy dielectric layer 210 may be deposited by an ALD process, a CVDprocess, a subatmospheric CVD (SACVD) process, a flowable CVD process, aPVD process, or other suitable process. By way of example, the dummydielectric layer 210 may be used to prevent damage to the semiconductorfins 112 and 114 by subsequent processing (e.g., subsequent formation ofthe dummy gate structure).

A dummy gate structure 220 is formed over the dummy dielectric layer210, the semiconductor fins 112, 114, and the isolation structures 120.In some embodiments, a dummy gate layer (not shown) may be formed overthe dummy dielectric layer 210, and is then patterned to form the dummygate electrode 220. In some embodiments, the dummy gate electrode 220may be made of polycrystalline-silicon (poly-Si), polycrystallinesilicon-germanium (poly-SiGe), or other suitable materials. If agate-first technology is employed, the dummy gate structure 220 and thedummy dielectric layer 210 are used as a gate electrode and a gatedielectric layer.

Reference is made to FIG. 3 . Portions of the dummy dielectric layer 210uncovered by the dummy gate structure 220 are removed to expose portionsof the semiconductor fins 112 and 114. Then, spacer structures 310 areat least formed on opposite sides of the dummy gate structure 220 andthe dummy dielectric layer 210. The spacer structures 310 may include aseal spacer and a main spacer (not shown). The spacer structures 310include one or more dielectric materials, such as silicon oxide, siliconnitride, silicon oxynitride, SiCN, SiCxOyNz, or combinations thereof.The seal spacers are formed on sidewalls of the dummy gate structure 220and the main spacers are formed on the seal spacers. The spacerstructures 310 can be formed using a deposition method, such as plasmaenhanced chemical vapor deposition (PECVD), low-pressure chemical vapordeposition (LPCVD), sub-atmospheric chemical vapor deposition (SACVD),or the like. The formation of the spacer structures 310 may includeblanket forming spacer layers and then performing etching operations toremove the horizontal portions of the spacer layers. The remainingvertical portions of the spacer layers form the spacer structures 310.In some embodiments, the isolation structures 120 are recessed when theetching operation of the spacer layers is performed, and the etchedamount H is in a range from about 0.5 nm to about 20 nm.

In some embodiments, spacer residues 312 and 314, which are remainingparts of the spacer structures 310 that is not removed in the operationof etching the spacer layer, exist. Specifically, in the operation ofthe spacer layer deposition process, the spacer layer also covers thesemiconductor fins 112 and 114. When the spacer layer is etched to formthe spacer structures 310, the portions of the spacer layer on sidewallsof the semiconductor fins 112 and 114 are pullback-etched. Portions ofthe spacer structures 310 thus remain at corners between the isolationstructure 120 and the semiconductor fins 112/114 after the etching andform the spacer residues 312/314. In some other embodiments, however,the spacer residues 312 and/or 314 may be omitted. The verticalthickness T1 of the spacer residue 312 is in a range from about 0.5 nmto about 30 nm in some embodiments. The vertical thickness T2 of thespacer residue 314 is in a range from about 0.5 nm to about 30 nm insome embodiments.

Reference is made to FIG. 4 . A first mask layer 410 is formed over thesecond region 104 of the substrate 110 while the first region 102 of thesubstrate 110 is exposed. That is, the semiconductor fins 112 areuncovered by the first mask layer 410 while the semiconductor fins 114are covered by the first mask layer 410. First epitaxial structures 420are then formed on portions of the semiconductor fins 112 uncovered bythe dummy gate structure 220, the spacer structures 310, and the firstmask layer 410 by performing, for example, a selectively growingprocess. The first epitaxial structures 420 are formed by epitaxiallygrowing a semiconductor material. The semiconductor material includessingle element semiconductor material, such as germanium (Ge) or silicon(Si), compound semiconductor materials, such as gallium arsenide (GaAs)or aluminum gallium arsenide (AlGaAs), or semiconductor alloy, such assilicon germanium (SiGe) or gallium arsenide phosphide (GaAsP). Thefirst epitaxial structures 420 have suitable crystallographicorientations (e.g., (110) and (111) crystallographic orientations), suchthat the first epitaxial structures 420 have hexagon cross sections. Forexample, the top surfaces 422 and the bottom surfaces 426 of the firstepitaxial structures 420 are (111) facets (i.e., top surfaces 422 areupward facing facets facing upwards, and the bottom surfaces 426 aredownward facing facets facing downwards), and the sidewalls 424 of thefirst epitaxial structures 420 are (110) facets (sidewall facets). Thefirst epitaxial structures 420 may be separated from each other as shownin FIG. 4 or be merged together. In some embodiments, the firstepitaxial structures 420 are source/drain epitaxial structures. In someembodiments, where an N-type device is desired, the first epitaxialstructures 420 may include an epitaxially growing silicon phosphorus(SiP) or silicon carbon (SiC). The epitaxial processes include CVDdeposition techniques (e.g., vapor-phase epitaxy (VPE) and/or ultra-highvacuum CVD (UHV-CVD)), molecular beam epitaxy, and/or other suitableprocesses.

In some embodiments, the first epitaxial structures 420 includes a firstepitaxial layer 432 formed on the semiconductor fin 112, a secondepitaxial layer 434 formed on the first epitaxial layer 432, and a thirdepitaxial layer 436 formed on the second epitaxial layer 434. The first,second, and third epitaxial layers 432, 434, 436 are crystallinesemiconductor layers, such as Si, SiC, SiCP, SiP, Ge and SiGe, havingdifferent lattice constants from each other and from the semiconductorfins 112. When SiC, SiP and/or SiCP are used, the C or P concentrationof the first epitaxial layer 432 is different from that of the secondand third epitaxial layers 434 and 436. In some embodiments, a GroupIII-V semiconductor layer is used for at least one of the first, second,and third epitaxial layers 432, 434, and 436. In some other embodiments,only one or two of the first, second, and third epitaxial layers 432,434, and 436 is formed, and in some other embodiments, more epitaxiallayers are formed.

Reference is made to FIG. 5 . The first mask layer 410 of FIG. 4 isremoved, and a second mask layer 510 is formed over the first region 102of the substrate 110 while the second region 104 of the substrate 110 isexposed. That is, the semiconductor fins 112 and the first epitaxialstructures 420 are covered by the second mask layer 510 while thesemiconductor fins 114 are uncovered by the second mask layer 510.Portions of the second fins 114 uncovered by the dummy gate structure220 and the spacer structures 310 are recessed, and second epitaxialstructures 520 are then formed on the recessed portion of thesemiconductor fins 114 by performing, for example, a selectively growingprocess. The second epitaxial structures 520 are formed by epitaxiallygrowing a semiconductor material. The semiconductor material includessingle element semiconductor material, such as germanium (Ge) or silicon(Si), compound semiconductor materials, such as gallium arsenide (GaAs)or aluminum gallium arsenide (AlGaAs), or semiconductor alloy, such assilicon germanium (SiGe) or gallium arsenide phosphide (GaAsP). Thesecond epitaxial structures 520 have suitable crystallographicorientations (e.g., a (100) crystallographic orientation), such that thesecond epitaxial structures 520 have diamond cross sections. In someembodiments, the top surfaces 522 and the bottom surfaces 526 of thesecond epitaxial structures 520 are (100) facets (i.e., top surfaces 522are upward facing facets facing upwards, and the bottom surfaces 526 aredownward facing facets facing downwards). The second epitaxialstructures 520 may be merged together as shown in FIG. 5 or be separatedfrom each other. In some embodiments, the second epitaxial structures520 include source/drain epitaxial structures. In some embodiments,where a P-type device is desired, the second epitaxial structures 520may include an epitaxially growing silicon germanium (SiGe). The firstepitaxial structures 420 and the second epitaxial structures 520 havedifferent conductivity types. The epitaxial processes include CVDdeposition techniques (e.g., vapor-phase epitaxy (VPE) and/or ultra-highvacuum CVD (UHV-CVD)), molecular beam epitaxy, and/or other suitableprocesses.

In some embodiments, the second epitaxial structures 520 includes afourth epitaxial layer 532 formed on the semiconductor fin 114, a fifthepitaxial layer 534 formed on the fourth epitaxial layer 532, and asixth epitaxial layer 536 formed on the fifth epitaxial layer 534. Thefourth, fifth, and sixth epitaxial layers 532, 534, and 536 arecrystalline semiconductor layers, such as Si, SiC, SiCP, SiP, Ge andSiGe, having different lattice constants from each other and from thesemiconductor fins 114. When SiGe are used, the Ge concentration of thefourth epitaxial layer 532 is different from that of the fifth and sixthepitaxial layers 534 and 536. In some embodiments, a Group III-Vsemiconductor layer is used for at least one of the fourth, fifth, andsixth epitaxial layers 532, 534, 536. In some other embodiments, onlyone or two of the fourth, fifth, and sixth epitaxial layers 532, 534,and 536 is formed, and in some other embodiments, more epitaxial layersare formed.

Reference is made to FIG. 6 . The second mask layer 510 of FIG. 5 isremoved. A first contact etch stop layer (CESL) 610 is then conformallyformed over the first epitaxial structures 420, the second epitaxialstructures 520, the dummy gate structure 220, the spacer structures 310,and the isolation structure 120. In some embodiments, the CESL 610 isnot formed under the merged portion of the second epitaxial structures520. In some embodiments, the first CESL 610 can be a stressed layer orlayers. In some embodiments, the first CESL 610 has a tensile stress andis formed of Si₃N₄. In some other embodiments, the first CESL 610includes materials such as oxynitrides. In yet some other embodiments,the first CESL 610 may have a composite structure including a pluralityof layers, such as a silicon nitride layer overlying a silicon oxidelayer. The first CESL 610 can be formed using plasma enhanced CVD(PECVD), however, other suitable methods, such as low pressure CVD(LPCVD), atomic layer deposition (ALD), and the like, can also be used.

A first interlayer dielectric (ILD) 620 is then formed on the first CESL610. The first ILD 620 may be formed by chemical vapor deposition (CVD),high-density plasma CVD, spin-on, sputtering, or other suitable methods.In some embodiments, the first ILD 620 includes silicon oxide. In someother embodiments, the first ILD 620 may include silicon oxy-nitride,silicon nitride, or a low-k material. Then, a planarization process,such as a chemical mechanical planarization (CMP) process, is performedto planarize the first CESL 610 and the first ILD 620 to expose thedummy gate structure 220.

Reference is made to FIG. 7 . A replacement gate (RPG) process scheme isemployed. In the RPG process scheme, a dummy polysilicon gate (the dummygate structure 220 (see FIG. 6 ) in this case) is formed in advance andis replaced later by a metal gate. In some embodiments, the dummy gatestructure 220 is removed to form an opening 702 with the spacerstructures 310 as its sidewalls. In some other embodiments, the dummydielectric layer 210 (see FIG. 6 ) is removed as well. Alternatively, insome embodiments, the dummy gate structure 220 is removed while thedummy dielectric layer 210 retains. The dummy gate structure 220 (andthe dummy dielectric layer 210) may be removed by dry etch, wet etch, ora combination of dry and wet etch.

A gate dielectric layer 712 is conformally formed in the opening 702.The gate dielectric layer 712 is over the semiconductor fins 112 and/or114. The gate dielectric layer 712 can be a high-κ dielectric layerhaving a dielectric constant (κ) higher than the dielectric constant ofSiO₂, i.e. κ>3.9. The gate dielectric layer 712 may include LaO, AlO,ZrO, TiO, Ta₂O₅, Y₂O₃, SrTiO₃ (STO), BaTiO₃ (BTO), BaZrO, HfZrO, HfLaO,HfSiO, LaSiO, AlSiO, HfTaO, HfTiO, (Ba,Sr)TiO₃ (BST), Al₂O₃, or othersuitable materials. The gate dielectric layer 712 is deposited bysuitable techniques, such as ALD, CVD, PVD, thermal oxidation,combinations thereof, or other suitable techniques.

At least one metal layer is formed in the opening 702 and on the gatedielectric layer 712. Subsequently, a chemical mechanical planarization(CMP) process is performed to planarize the metal layer and the gatedielectric layer 712 to form metal gate stack 710 in the opening 702.The metal gate stack 710 crosses over the semiconductor fins 112 and/or114. The metal gate stack 710 includes the gate dielectric layer 712 anda metal gate electrode 714 over the gate dielectric layer 712. The metalgate electrode 714 may include work function metal layer(s), cappinglayer(s), fill layer(s), and/or other suitable layers that are desirablein a metal gate stack. The work function metal layer may include n-typeand/or p-type work function metal. Exemplary n-type work function metalsinclude Ti, Ag, TaAl, TaAlC, TiAlN, TaC, TaCN, TaSiN, Mn, Zr, othersuitable n-type work function materials, or combinations thereof.Exemplary p-type work function metals include TiN, TaN, Ru, Mo, Al, WN,ZrSi₂, MoSi₂, TaSi₂, NiSi₂, other suitable p-type work functionmaterials, or combinations thereof. The work function metal layer mayhave multiple layers. The work function metal layer(s) may be depositedby CVD, PVD, electroplating and/or other suitable process. In someembodiments, the metal gate electrode 714 is a p-type metal gateincluding a p-type work function metal layer. In some embodiments, thecapping layer in the metal gate electrodes 714 may include refractorymetals and their nitrides (e.g. TiN, TaN, W₂N, TiSiN, TaSiN). Thecapping layer may be deposited by PVD, CVD, metal-organic chemical vapordeposition (MOCVD), ALD, or the like. In some embodiments, the filllayer in the metal gate electrodes 714 may include tungsten (W). Thefill layer may be deposited by ALD, PVD, CVD, or other suitable process.

Reference is made to FIGS. 8A and 8B. FIG. 8B is a cross-sectional viewtaken along line B-B of FIG. 8A. The first ILD 620 of FIG. 7 is removedto expose the first CESL 610. In some embodiments, the first ILD 620 isfully removed. In some other embodiments, the first ILD 620 is partiallyremoved from areas around the first epitaxial structures 420 and thesecond epitaxial structures 520. Then, portions of the first CESL 610over the first epitaxial structures 420 and the second epitaxialstructures 520 are removed to expose the top surfaces 422 of the firstepitaxial structures 420 and the top surfaces 522 of the secondepitaxial structures 520. The sidewalls 424 and the bottom surfaces 426of the first epitaxial structures 420 and the bottom surfaces 526 of thesecond epitaxial structures 520 are still covered by the CESL 610. Insome embodiments, the first CESL 610 is anisotropic etched byperforming, for example, a reactive ion etch (RIE) process or othersuitable processes. Anisotropic etching means different etch rates indifferent directions in the material. That is, an anisotropic etchingremoves the material being etched at different rates in differentdirections. For example, in FIGS. 8A and 8B, the anisotropic etchingremoves the horizontal portions of the first CESL 610 faster than thevertical portions thereof. As such, the portions of the first CESL 610over the top surfaces 422 and 522 are removed and other portions of thefirst CESL 610 remains. Furthermore, the portions of the CESL 610 overthe isolation structures 120 are also removed in this stage.

Reference is made to FIG. 9 . FIG. 9 is taken along the same line asFIG. 8B. A metal material is directionally (or anisotropically) formedover the structure of FIG. 8B, such that a first metal layer 910 and asecond metal layer 920 are respectively formed over the top surfaces 422and 522. The anisotropic deposition method employed to deposit the metalmaterial can be methods that provide a directional deposition so thatmore metal material is deposited on horizontal surfaces than on verticalsurfaces. For example, the anisotropic deposition method can be acollimated physical vapor deposition (PVD) method, in which the metalmaterial is directed downward in directions substantially parallel tothe vertical direction of the exemplary semiconductor structure. Theterm “substantially” as used herein may be applied to modify anyquantitative representation which could permissibly vary withoutresulting in a change in the basic function to which it is related.Alternately, the anisotropic deposition method can employ radiofrequency physical vapor deposition (RFPVD) sputtering and/or withconstant voltage substrate bias, i.e., constant electrical voltage biasapplied to the substrate. Also, the anisotropic deposition method may bean ion CVD, or other suitable processes. The deposition rate depends onthe angle of incidence of incoming particles, resulting in a higherdeposition rate on the top surfaces 422 than surfaces of the first CESL610. Hence, the first metal layer 910 is in contact with the topsurfaces 422 and not in contact with other surfaces of the firstepitaxial structures 420, and the second metal layer 920 is in contactwith the top surfaces 522 and not in contact with other surfaces of thesecond epitaxial structures 520. In some embodiments, portions of themetal material are formed over the isolation structures 120 to form anexcess metal layer 930. The metal material (i.e., the first metal layer910, the second metal layer 920, and the excess metal layer 930) is madeof Ni, Co, Pt, W, Ru, combinations thereof, or other suitable materials.The first metal layer 910 and the second metal layer 920 have high workfunction, for example, in a range from about 4.4 eV to about 5.2 eV.

Reference is made to FIG. 10 . An annealing process is performed on thefirst metal layer 910 (see FIG. 9 ), the second metal layer 920 (seeFIG. 9 ), the first epitaxial structures 420, and the second epitaxialstructures 520 to form a first top metal alloy layer 1010 and a secondtop metal alloy layer 1020 respectively. The annealing process is alsoreferred to as a silicide process if the first epitaxial structures 420and the second epitaxial structures 520 are made of silicon. Thesilicide process converts the surface portions of the first epitaxialstructures 420 and the second epitaxial structures 520 into silicidecontacts (i.e., the first top metal alloy layer 1010 and the second topmetal alloy layer 1020 in this case). Silicide processing involvesdeposition of a metal material (i.e., the first metal layer 910 and thesecond metal layer 920 in this case) that undergoes a silicidationreaction with silicon (Si). In order to form silicide contacts on thefirst epitaxial structures 420 and the second epitaxial structures 520,the first metal layer 910 and the second metal layer 920 arerespectively blanket deposited on the top surfaces 422 of the firstepitaxial structures 420 and the top surfaces 522 of the secondepitaxial structures 520. After heating the wafer to a temperature atwhich the metal reacts with the silicon of the first epitaxialstructures 420 and the second epitaxial structures 520 to form contacts,unreacted metal (such as the excess metal layer 930 of FIG. 9 ) isremoved. The silicide contacts remain over the first epitaxialstructures 420 and the second epitaxial structures 520, while unreactedmetal is removed from other areas.

Reference is made to FIG. 11 . The remaining first CESL 610 of FIG. 10is removed, such that the sidewalls 424 and the bottom surfaces 426 ofthe first epitaxial structures 420 and the bottom surfaces 526 of thesecond epitaxial structures 520 are exposed. Moreover, the top surfaces422 are covered by the first top metal alloy layer 1010 and the topsurfaces 522 are covered by the second top metal alloy layer 1020. Insome embodiments, the first CESL 610 is isotropic etched by performing,for example, a wet chemical etching process or other suitable processes.An “isotropic etching” is an etching process that is not a directionaletching. An isotropic etching removes the material being etched at thesubstantially same rate in each direction. That is, isotropic etchingdoes not etch in a single direction, but rather etches horizontally aswell as vertically into the first CESL 610.

Reference is made to FIG. 12 . Another metal material is conformally (ornon-directionally) formed over the structure of FIG. 11 , such that athird metal layer 1210 and a fourth metal layer 1220 are respectivelyformed on the first epitaxial structures 420 and the second epitaxialstructures 520. That is, the third metal layer 1210 is in contact withthe first top metal alloy layer 1010 and the sidewalls 424 and thebottom surfaces 426 of the first epitaxial structures 420, and thefourth metal layer 1220 is in contact with the second top metal alloylayer 1020 and the bottom surfaces 526 of the second epitaxialstructures 520. In some embodiments, the fourth metal layer 1220 is notformed under the merged portion of the second epitaxial structures 420.The third metal layer 1210 and the fourth metal layer 1220 are made ofTi, Er, Y, Yb, Eu, Tb, Lu, Th, Sc, Hf, Zr, Ta, combinations thereof, orother suitable materials. The third metal layer 1210 and the fourthmetal layer 1220 have low work function, for example, in a range fromabout 2.5 eV to about 4.4 eV. That is, the third metal layer 1210 andthe fourth metal layer 1220 have a work function lower than that of thefirst metal layer 910 and the second metal layer 920 as shown in FIG. 9. The third metal layer 1210 and the fourth metal layer 1220 are formedby performing a conformally (or isotropic) deposition process, such asPECVD, PEALD, or other suitable processes. That is, the metal materialis deposited over not only one direction, but different directions ofthe first epitaxial structures 420 and the second epitaxial structures520.

Reference is made to FIG. 13 . Another annealing process is performed onthe first top metal alloy layer 1010 (see FIG. 12 ), the second topmetal alloy layer 1020 (see FIG. 12 ), the third metal layer 1210 (seeFIG. 12 ), the fourth metal layer 1220 (see FIG. 12 ), the firstepitaxial structures 420, and the second epitaxial structures 520. Thefirst top metal alloy layer 1010, the third metal layer 1210, and thefirst epitaxial structures 420 are annealed to form a third top metalalloy layer 1310, the second top metal alloy layer 1020, the fourthmetal layer 1220, and the second epitaxial structures 520 are annealedto form a fourth top metal alloy layer 1320, the third metal layer 1210and the first epitaxial structures 420 are annealed to form a firstbottom metal alloy layer 1330, and the fourth metal layer 1220 and thesecond epitaxial structures 520 are annealed to form a second bottommetal alloy layer 1340. The annealing process is also referred to as asilicide process if the first epitaxial structures 420 and the secondepitaxial structures 520 are made of silicon. After the annealingprocess, the unreacted metal is removed. The third top metal alloy layer1310 is in contact with the top surfaces 422 of the first epitaxialstructures 420, the fourth top metal alloy layer 1320 is in contact withthe top surfaces 522 of the second epitaxial structures 520, the firstbottom metal alloy layer 1330 is in contact with the sidewalls 424 andthe bottom surfaces 426 of the first epitaxial structures 420, and thesecond bottom metal alloy layer 1330 is in contact with the bottomsurfaces 526 of the second epitaxial structures 520.

In some embodiments, the third top metal alloy layer 1310 and the fourthtop metal alloy layer 1320 include the high WF metals including Ni, Co,Pt, W, Ru, or combinations thereof and the low WF metals including Ti,Er, Y, Yb, Eu, Tb, Lu, Th, Sc, Hf, Zr, Tb, Ta, or combination thereof,and the first bottom metal alloy layer 1330 and the second bottom metalalloy layer 1340 include the low WF metals including Ti, Er, Y, Yb, Eu,Tb, Lu, Th, Sc, Hf, Zr, Tb, Ta, or combinations thereof. As such, thethird top metal alloy layer 1310 and the fourth top metal alloy layer1320 have higher work function than the first bottom metal alloy layer1330 and the second bottom metal alloy layer 1340. For the firstepitaxial structures 420, the contact areas between the first bottommetal alloy layer 1330 and the first epitaxial structures 420 is largerthan the contact areas between the third top metal alloy layer 1310 andthe first epitaxial structures 420, such that the equivalent workfunction of the source/drain feature of N-type device (i.e., the firstepitaxial structures 420, the first bottom metal alloy layer 1330, andthe third top metal alloy layer 1310 in this case) is between the workfunctions of the first bottom metal alloy layer 1330 and the third topmetal alloy layer 1310 but close to the first bottom metal alloy layer1330. For the second epitaxial structures 520, the contact areas betweenthe fourth top metal alloy layer 1320 and the second epitaxialstructures 520 is larger than the contact areas between the secondbottom metal alloy layer 1340 and the second epitaxial structures 520,such that the equivalent work function of the source/drain feature ofP-type device (i.e., the second epitaxial structures 520, the secondbottom metal alloy layer 1340, and the fourth top metal alloy layer 1320in this case) is between the work functions of the second bottom metalalloy layer 1340 and the fourth top metal alloy layer 1320 but close tothe fourth top metal alloy layer 1320. Therefore, the source/drainfeatures of N-type and P-type devices have different work functions. Insome embodiments, the third top metal alloy layer 1310 has a thicknessin a range from about 2 nm to about 7 nm, the fourth top metal alloylayer 1320 has a thickness in a range from about 2 nm to about 7 nm, thefirst bottom metal alloy layer 1330 has a thickness in a range fromabout 2 nm to about 6 nm, and the second bottom metal alloy layer 1340has a thickness in a range from about 2 nm to about 6 nm. If thethicknesses of the layers 1310, 1320, 1330, and 1340 are too small, suchas less than about 2 nm, the schottky barrier of the source/drainfeatures is affected, and the electrical properties of the source/drainfeatures become worse.

Reference is made to FIG. 14 . A second contact etch stop layer (CESL)1410 is conformally formed over the structure of FIG. 13 . In someembodiments, the CESL 1410 is not formed under the merged portion of thesecond epitaxial structures 520. In some embodiments, the second CESL1410 can be a stressed layer or layers. In some embodiments, the secondCESL 1410 has a tensile stress and is formed of Si₃N₄. In some otherembodiments, the second CESL 1410 includes materials such asoxynitrides. In yet some other embodiments, the second CESL 1410 mayhave a composite structure including a plurality of layers, such as asilicon nitride layer overlying a silicon oxide layer. The second CESL1410 can be formed using plasma enhanced CVD (PECVD), however, othersuitable methods, such as low pressure CVD (LPCVD), atomic layerdeposition (ALD), and the like, can also be used.

A second interlayer dielectric (ILD) 1420 is then formed on the secondCESL 1410. The second ILD 1420 may be formed by chemical vapordeposition (CVD), high-density plasma CVD, spin-on, sputtering, or othersuitable methods. In some embodiments, the second ILD 1420 includessilicon oxide. In some other embodiments, the second ILD 1420 mayinclude silicon oxy-nitride, silicon nitride, or a low-k material.

Reference is made to FIG. 15 . The second ILD 1420 and the second CESL1410 are partially removed to form a plurality of openings 1422 and 1424by various methods, including a dry etch, a wet etch, or a combinationof dry etch and wet etch. The openings 1422 and 1424 extend through thesecond ILD 1420 and the second CESL 1410 and respectively expose thethird top metal alloy layer 1310 and the fourth top metal alloy layer1320.

Contacts 1510 and 1520 are respectively formed in the openings 1422 and1424 and respectively over the third top metal alloy layer 1310 and thefourth top metal alloy layer 1320. The contacts 1510 and 1520 arerespectively and electrically connected to the first epitaxialstructures 420 and the second epitaxial structures 520. The contact 1510includes a barrier layer 1512 and a filling material 1514 formed overthe barrier layer 1512. The contact 1520 includes a barrier layer 1522and a filling material 1524 formed over the barrier layer 1522. In someembodiments, metal materials can be filled in the openings 1422 and1424, and excessive portions of the metal materials are removed byperforming a planarization process to form the filling materials 1514and 1524. In some embodiments, the barrier layers 1512 and 1522 mayinclude one or more layers of a material such as, for example, titanium,titanium nitride, titanium tungsten or combinations thereof. In someembodiments, the filling materials 1514 and 1524 may be made of, forexample, tungsten, aluminum, copper, or other suitable materials. Insome embodiments, the depth of the contact 1510/1520 is in a range fromabout 15 nm to about 60 nm, depending on node and the semiconductor finheights.

In FIG. 15 , the third top metal alloy layers 1310 and the first bottommetal alloy layers 1330 are formed on the third epitaxial layer 436. Thethird top metal alloy layers 1310 and the first bottom metal alloylayers 1330 are formed by a reaction between the material of the thirdepitaxial layer 436 and metal layers formed thereon. The third epitaxiallayer 436 of one of the first epitaxial structures 420 is separated fromthe third epitaxial layer 436 of the other one of the first epitaxialstructure 420. In some embodiments, the first bottom metal alloy layers1330 are separated from each other as shown in FIG. 15 ; in some otherembodiments, the first bottom metal alloy layers 1330 fills the spacebetween the two first epitaxial structures 420; in still some otherembodiments, one or more void(s) and/or hole(s) is(are) formed in thefirst bottom metal alloy layers 1330 and between the two first epitaxialstructures 420. The shapes of the voids in the cross section may includea rhombus, a circle, an oval, or an irregular shape. The shapes may besymmetry or asymmetric. The number of the voids may be as small as onein some embodiments, and more than one in some other embodiments. Sizesof the multiple voids and spaces between voids may be substantially thesame or different.

In some embodiments, the semiconductor fin 112 has a width (thickness)W1 in a range from about 4 nm to about 10 nm; the first epitaxial layer432 has a width (thickness) W2 in a range from about 0 nm to about 3 nm;the second epitaxial layer 434 has a width (thickness) W3 in a rangefrom about 2 nm to about 8 nm; and the third epitaxial layer 436 has awidth (thickness) W4 in a range from about 0 nm to about 3 nm.

Further, the fourth top metal alloy layers 1320 and the second bottommetal alloy layers 1340 are formed on the sixth epitaxial layer 536. Thefourth top metal alloy layers 1320 and the second bottom metal alloylayers 1340 are formed by a reaction between the material of the sixthepitaxial layer 536 and metal layers formed thereon. As shown in FIG. 15, the sixth epitaxial layer 536 of one of the second epitaxialstructures 520 is merged with the sixth epitaxial layer 536 of the otherone of the second epitaxial structure 520.

In some embodiments, the fourth epitaxial layer 532 has a thickness(height) T3 in a range from about 0 nm to about 3 nm; the fifthepitaxial layer 534 has a thickness (height) T4 in a range from about 2nm to about 8 nm; and the sixth epitaxial layer 536 has a thickness T5in a range from about 0 nm to about 3 nm. Moreover, the width W5 of thefourth epitaxial layer 532 may be greater than, less than, or equal tothe thickness T3 of the fourth epitaxial layer 532. For example,T3:W5=1:1.2 to 1:2 in some embodiments. In some embodiments, the spacebetween two semiconductor fins 114 is greater than about 15 nm.

Furthermore, it is noted although in FIG. 15 , the spacer residues 314formed on opposite sides of the semiconductor fin 114 have the sameheight, the spacer residues 314 may have different heights in some otherembodiments. For example, the spacer residue 314 i formed between twosemiconductor fins 114 is higher than the spacer residue 314 i formed onopposite side of the semiconductor fins 114. This is because the densespace between the two semiconductor fins 114 results in slow etchingrate during the etching operations in FIG. 3 . In some embodiments, thedifference between the spacer residues 314 i and 314 o is in a rangefrom 0 nm to about 10 nm.

In FIG. 15 , the first epitaxial structure 420 is in contact with thethird top metal alloy layer 1310 and the first bottom metal alloy layer1330. As such, the equivalent work function of the first epitaxialstructure 420 can be tuned by applying different materials of the thirdtop metal alloy layer 1310 and the first bottom metal alloy layer 1330.Furthermore, the second epitaxial structure 520 is in contact with thefourth top metal alloy layer 1320 and the second bottom metal alloylayer 1340. Since the first epitaxial structure 420 and the secondepitaxial structure 520 have different cross sections, the source/drainfeatures of the devices on the regions 102 and 104 have differentequivalent work functions even though the third and fourth top metalalloy layers 1310 and 1320 have the same work function and the first andsecond bottom metal alloy layers 1330 and 1340 have the same workfunction. As such, the N-type and P-type devices can achieve theirdesired source/drain equivalent work functions in the same metal alloylayers formation process and without additional photo patterning flows.

FIGS. 16-22 illustrate a method for manufacturing a semiconductor deviceat various stages in accordance with some embodiments of the presentdisclosure. The manufacturing processes of FIGS. 1 to 7 are performed inadvance. Since the relevant manufacturing details are similar to FIGS. 1to 7 , and, therefore, a description in this regard will not be repeatedhereinafter. Reference is made to FIG. 16 . The first ILD 620 of FIG. 7is removed to expose the first CESL 610 of FIG. 7 . Then, the first CESL610 is removed to expose the first epitaxial structures 420 and thesecond epitaxial structures 520. In this stage, the top surfaces 422,the sidewalls 424, and the bottom surfaces 426 of the first epitaxialstructures 420 and the top surfaces 522 and the bottom surfaces 526 ofthe second epitaxial structures 520 are exposed.

Reference is made to FIG. 17 . A metal material is directionally (oranisotropically) formed over the structure of FIG. 16 , such that afirst metal layer 910 and a second metal layer 920 are respectivelyformed over the top surfaces 422 and 522. That is, the first metal layer910 is in contact with the top surfaces 422 and not in contact withother surfaces of the first epitaxial structures 420, and the secondmetal layer 920 is in contact with the top surfaces 522 and not incontact with other surfaces of the second epitaxial structures 520. Insome embodiments, portions of the metal material are formed over theisolation structures 120 to form the excess metal layer 930. The firstmetal layer 910 and the second metal layer 920 are made of Ni, Co, Pt,W, Ru, combinations thereof, or other suitable materials. The firstmetal layer 910 and the second metal layer 920 have high work function,for example, in a range from about 4.4 eV to about 5.2 eV. The firstmetal layer 910 and the second metal layer 920 are formed by performinga directionally deposition process, such as PVD, REPVD, ion CVD, orother suitable processes.

Reference is made to FIG. 18 . An annealing process is performed on thefirst metal layer 910 (see FIG. 17 ), the second metal layer 920 (seeFIG. 17 ), the first epitaxial structures 420, and the second epitaxialstructures 520 to form a first top metal alloy layer 1010 and a secondtop metal alloy layer 1020 respectively. The annealing process is alsoreferred to as a silicide process if the first epitaxial structures 420and the second epitaxial structures 520 are made of silicon. After theannealing process, the unreacted metal (such as the excess metal layer930 of FIG. 17 ) is removed.

Reference is made to FIG. 19 . Another metal material is conformally (orisotropically) formed over the structure of FIG. 18 , such that a thirdmetal layer 1210 and a fourth metal layer 1220 are respectively formedon the first epitaxial structures 420 and the second epitaxialstructures 520. That is, the third metal layer 1210 is in contact withthe first top metal alloy layer 1010 and the sidewalls 424 and thebottom surfaces 426 of the first epitaxial structures 420, and thefourth metal layer 1220 is in contact with the second top metal alloylayer 1020 and the bottom surfaces 526 of the second epitaxialstructures 520. In some embodiments, the fourth metal layer 1220 is notformed under the merged portion of the second epitaxial structures 420.The third metal layer 1210 and the fourth metal layer 1220 are made ofTi, Er, Y, Yb, Eu, Tb, Lu, Th, Sc, Hf, Zr, Tb, Ta, combinations thereof,or other suitable materials. The third metal layer 1210 and the fourthmetal layer 1220 have low work function, for example, in a range fromabout 2.5 eV to about 4.4 eV. That is, the third metal layer 1210 andthe fourth metal layer 1220 have a work function lower than that of thefirst metal layer 910 and the second metal layer 920 as shown in FIG. 9. The third metal layer 1210 and the fourth metal layer 1220 are formedby performing a conformally deposition process, such as PECVD, PEALD, orother suitable processes.

Reference is made to FIG. 20 . Another annealing process is performed onthe first top metal alloy layer 1010 (see FIG. 19 ), the second topmetal alloy layer 1020 (see FIG. 19 ), the third metal layer 1210 (seeFIG. 19 ), the fourth metal layer 1220 (see FIG. 19 ), the firstepitaxial structures 420, and the second epitaxial structures 520. Thefirst top metal alloy layer 1010, the third metal layer 1210, and thefirst epitaxial structures 420 are annealed to form a third top metalalloy layer 1310, the second top metal alloy layer 1020, the fourthmetal layer 1220, and the second epitaxial structures 520 are annealedto form a fourth top metal alloy layer 1320, the third metal layer 1210and the first epitaxial structures 420 are annealed to form a firstbottom metal alloy layer 1330, and the fourth metal layer 1220 and thesecond epitaxial structures 520 are annealed to form a second bottommetal alloy layer 1340. The annealing process is also referred to as asilicide process if the first epitaxial structures 420 and the secondepitaxial structures 520 are made of silicon. After the annealingprocess, the unreacted metal is removed. The third top metal alloy layer1310 is in contact with the top surfaces 422 of the first epitaxialstructures 420, the fourth top metal alloy layer 1320 is in contact withthe top surfaces 522 of the second epitaxial structures 520, the firstbottom metal alloy layer 1330 is in contact with the sidewalls 424 andthe bottom surfaces 426 of the first epitaxial structures 420, and thesecond bottom metal alloy layer 1330 is in contact with the bottomsurfaces 526 of the second epitaxial structures 520.

In some embodiments, the third top metal alloy layer 1310 and the fourthtop metal alloy layer 1320 are made of a material including the high WFmetals including Ni, Co, Pt, W, Ru, or combinations thereof and the lowWF metals including Ti, Er, Y, Yb, Eu, Tb, Lu, Th, Sc, Hf, Zr, Tb, Ta,or combination thereof, and the first bottom metal alloy layer 1330 andthe second bottom metal alloy layer 1340 include the low WF metalsincluding Ti, Er, Y, Yb, Eu, Tb, Lu, Th, Sc, Hf, Zr, Tb, Ta, orcombinations thereof. As such, the third top metal alloy layer 1310 andthe fourth top metal alloy layer 1320 have higher work function than thefirst bottom metal alloy layer 1330 and the second bottom metal alloylayer 1340. For the first epitaxial structures 420, the contact areasbetween the first bottom metal alloy layer 1330 and the first epitaxialstructures 420 is larger than the contact areas between the third topmetal alloy layer 1310 and the first epitaxial structures 420, such thatthe equivalent work function of the source/drain feature of N-typedevice (i.e., the first epitaxial structures 420, the first bottom metalalloy layer 1330, and the third top metal alloy layer 1310 in this case)is between the work functions of the first bottom metal alloy layer 1330and the third top metal alloy layer 1310 but close to the first bottommetal alloy layer 1330. For the second epitaxial structures 520, thecontact areas between the fourth top metal alloy layer 1320 and thesecond epitaxial structures 520 is larger than the contact areas betweenthe second bottom metal alloy layer 1340 and the second epitaxialstructures 520, such that the equivalent work function of thesource/drain feature of P-type device (i.e., the second epitaxialstructures 520, the second bottom metal alloy layer 1340, and the fourthtop metal alloy layer 1320 in this case) is between the work functionsof the second bottom metal alloy layer 1340 and the fourth top metalalloy layer 1320 but close to the fourth top metal alloy layer 1320.Therefore, the source/drain features of N-type and P-type devices havedifferent work functions.

Reference is made to FIG. 21 . A second contact etch stop layer (CESL)1410 is conformally formed over the structure of FIG. 20 . In someembodiments, the CESL 1410 is not formed under the merged portion of thesecond epitaxial structures 520. In some embodiments, the second CESL1410 can be a stressed layer or layers. In some embodiments, the secondCESL 1410 has a tensile stress and is formed of Si₃N₄. In some otherembodiments, the second CESL 1410 includes materials such asoxynitrides. In yet some other embodiments, the second CESL 1410 mayhave a composite structure including a plurality of layers, such as asilicon nitride layer overlying a silicon oxide layer. The second CESL1410 can be formed using plasma enhanced CVD (PECVD), however, othersuitable methods, such as low pressure CVD (LPCVD), atomic layerdeposition (ALD), and the like, can also be used.

A second interlayer dielectric (ILD) 1420 is then formed on the secondCESL 1410. The second ILD 1420 may be formed by chemical vapordeposition (CVD), high-density plasma CVD, spin-on, sputtering, or othersuitable methods. In some embodiments, the second ILD 1420 includessilicon oxide. In some other embodiments, the second ILD 1420 mayinclude silicon oxy-nitride, silicon nitride, or a low-k material.

Reference is made to FIG. 22 . The second ILD 1420 and the second CESL1410 are partially removed to form a plurality of openings 1422 and 1424by various methods, including a dry etch, a wet etch, or a combinationof dry etch and wet etch. The openings 1422 and 1424 extend through thesecond ILD 1420 and the second CESL 1410 and respectively expose thethird top metal alloy layer 1310 and the fourth top metal alloy layer1320.

Contacts 1510 and 1520 are respectively formed in the openings 1422 and1424 and respectively over the third top metal alloy layer 1310 and thefourth top metal alloy layer 1320. The contacts 1510 and 1520 arerespectively and electrically connected to the first epitaxialstructures 420 and the second epitaxial structures 520. The contact 1510includes a barrier layer 1512 and a filling material 1514 formed overthe barrier layer 1512. The contact 1520 includes a barrier layer 1522and a filling material 1524 formed over the barrier layer 1522. In someembodiments, metal materials can be filled in the openings 1422 and1424, and excessive portions of the metal materials are removed byperforming a planarization process to form the filling materials 1514and 1524. In some embodiments, the barrier layers 1512 and 1522 mayinclude one or more layers of a material such as, for example, titanium,titanium nitride, titanium tungsten or combinations thereof. In someembodiments, the filling materials 1514 and 1524 may be made of, forexample, tungsten, aluminum, copper, or other suitable materials.

In some embodiments, the N-type device (the device over the region 102)in FIGS. 15 and 22 can be an N-type target critical dimension (TCD)device, and the P-type device (the device over the region 104) in FIGS.15 and 22 can be a P-type TCD device. FIG. 23 is a cross-sectional viewof a semiconductor device in accordance with some embodiments of thepresent disclosure. The semiconductor device includes an N-type deviceover a first region 102′ of the substrate 110 and P-type devices over asecond region 104′ of the substrate 110. The semiconductor device may bea part of a SRAM device. The P-type devices can be pull-up (PU)transistors and the N-type device can be a pull-down (PD) transistor ora pass gate (PG) transistor. It is noted that the numbers of the N-typedevice and P-type devices in FIG. 23 are illustrative, and should notlimit the claimed scope of the present disclosure.

The N-type device includes a first epitaxial structure 420′ and a gatestructure (e.g., the metal gate stack 710 shown in FIG. 7 ) adjacent thefirst epitaxial structure 420′, and the P-type device includes secondepitaxial structures 520′ and a gate structure (e.g., the metal gatestack 710 shown in FIG. 7 ) adjacent the second epitaxial structures520′. The first epitaxial structure 420′ includes a first epitaxiallayer 434′ formed on the semiconductor fin 112 and a second epitaxiallayer 436′ formed on the first epitaxial layer 434′. The first andsecond epitaxial layers 434′ and 436′ are crystalline semiconductorlayers, such as Si, SiC, SiCP, SiP, Ge and SiGe, having differentlattice constants from each other and from the semiconductor fins 112.When SiC, SiP and/or SiCP are used, the C or P concentration of thefirst epitaxial layer 434′ is different from that of the secondepitaxial layer 436′. In some embodiments, a Group III-V semiconductorlayer is used for at least one of the first and second epitaxial layers434′ and 436′. In some other embodiments, only one of the first andsecond epitaxial layers 434′ and 436′ is formed, and in some otherembodiments, more epitaxial layers are formed. In some embodiments, alateral extension distance L1 of the first epitaxial structures 420′extending from a sidewall of the semiconductor fin 112 is less thanabout 7 nm.

The third top metal alloy layer 1310 and the first bottom metal alloylayer 1330 are formed on the second epitaxial layer 436′. The third topmetal alloy layer 1310 and the first bottom metal alloy layer 1330 areformed by a reaction between the material of the second epitaxial layer436′ and metal layers formed thereon.

The second epitaxial structures 520′ each include a third epitaxiallayer 534′ formed on the semiconductor fin 114 and a fourth epitaxiallayer 536′ formed on the third epitaxial layer 534′. The third andfourth epitaxial layers 534′ and 536′ are crystalline semiconductorlayers, such as Si, SiC, SiCP, SiP, Ge and SiGe, having differentlattice constants from each other and from the semiconductor fins 112.When SiGe are used, the Ge concentration of the third epitaxial layer534′ is different from that of the fourth epitaxial layer 536′. In someembodiments, a Group III-V semiconductor layer is used for at least oneof the third and fourth epitaxial layers 534′ and 536′. In some otherembodiments, only one of the third and fourth epitaxial layers 534′ and536′ is formed, and in some other embodiments, more epitaxial layers areformed. In some embodiments, a lateral extension distance L2 of thesecond epitaxial structures 520′ extending from a sidewall of thesemiconductor fin 114 is less than about 7 nm. Furthermore, in someembodiments, the spacer residues 316 are formed on the sidewalls of thesemiconductor fins 114, and the height Hs of the spacer residues 316 isin a range of 0 nm to about ⅔ of the fin height. The height Hs of thespacer residues 316 is associated with the size of the second epitaxialstructures 520′.

Further, the fourth top metal alloy layers 1320 and the second bottommetal alloy layers 1340 are formed on the fourth epitaxial layer 536′.The fourth top metal alloy layers 1320 and the second bottom metal alloylayers 1340 are formed by a reaction between the material of the fourthepitaxial layer 536′ and metal layers formed thereon. As shown in FIG.23 , the fourth epitaxial layer 536′ of one of the second epitaxialstructures 520′ is separated from the fourth epitaxial layer 536′ of theother one of the second epitaxial structure 520′. For example, adistance D1 between the second epitaxial structures 520′ is in a rangefrom about 10 nm to about 15 nm, and a distance D2 between thesemiconductor fins 114 is in a range from about 15 nm to about 30 nm. Ifthe distance D1 is less than about 10 nm and/or the distance D2 is lessthan about 15 nm, the second epitaxial structure 520′ may be merged. Ifthe distance D2 is greater than about 15 nm and/or the distance D2 isgreater than about 30 nm, the layout area of the semiconductor devicemay be increased. Moreover, the surface 526′ and the top surface of thesubstrate 110 form an angle θ in a range from about 35 degrees to about60 degrees, e.g., about 40 degrees or about 54.7 degrees.

According to some embodiments, the first epitaxial structure is incontact with the third top metal alloy layer and the first bottom metalalloy layer. As such, the equivalent work function of the firstepitaxial structure can be tuned by applying different materials of thethird top metal alloy layer and the first bottom metal alloy layer.Furthermore, the second epitaxial structure is in contact with thefourth top metal alloy layer and the second bottom metal alloy layer.For a semiconductor device including different types (e.g., N-type andP-type) devices, since the first epitaxial structure and the secondepitaxial structure have different cross sections, the source/drainfeatures of the N-type and P-type devices have different equivalent workfunctions even though the third and fourth top metal alloy layers havethe same work function and the first and second bottom metal alloylayers have the same work function. As such, the N-type and P-typedevices can achieve their desired source/drain equivalent work functionsin the same metal alloy layers formation process and without additionalphoto patterning flows.

According to some embodiments, a semiconductor device includes first andsecond epitaxial structures, first and second top metal alloy layers,and first and second bottom metal alloy layers. The first and secondepitaxial structures have different cross sections. The first and secondtop metal alloy layers are respectively in contact with the first andsecond epitaxial structures. The first and second bottom metal alloylayers are respectively in contact with the first and second epitaxialstructures and respectively under the first and second top metal alloylayers. The first top metal alloy layer and the first bottom metal alloylayer are made of different materials.

According to some embodiments, a semiconductor device includes a firstepitaxial structure, a second epitaxial structure, first and second topmetal alloy layers, and first and second bottom metal alloy layers. Thefirst epitaxial structure has an upward facing facet facing upwards anda downward facing facet facing downwards. The second epitaxial structurehas an upward facing facet facing upwards and a downward facing facetfacing downwards. The first epitaxial structure and the second epitaxialstructure have different conductivity types. The first and second topmetal alloy layers are respectively in contact with the upward facingfacet of the first epitaxial structure and the upward facing facet ofthe second epitaxial structure. The first and second bottom metal alloylayers are respectively in contact with the downward facing facet of thefirst epitaxial structure and the downward facing facet of the secondepitaxial structure. The first top metal alloy layer and the firstbottom metal alloy layer have different work functions.

According to some embodiments, a method for manufacturing asemiconductor device includes forming first and second epitaxialstructures over a substrate, wherein the first and second epitaxialstructures have different conductivity types, and the first and secondepitaxial structures each has an upward facing facet facing upwards anda downward facing facet facing downwards. First and second metal layersare formed respectively on the upward facing facets of the first andsecond epitaxial structures. The first and second metal layers and thefirst and second epitaxy structures are annealed to form a first topmetal alloy layer on the upward facing facet of the first epitaxialstructure and a second top metal alloy layer on the upward facing facetof the second epitaxial structure. A third metal layer is formed atleast on the downward facing facets of the first and second epitaxialstructures. The first and third metal layers have different metals. Thethird metal layer and the first and second epitaxial structures areannealed to form a first bottom metal alloy layer on the downward facingfacet of the first epitaxial structure, and a second bottom metal alloylayer on the downward facing facet of the second epitaxial structure.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor device comprising: asemiconductive substrate; a semiconductive fin protruding from thesemiconductive substrate; an isolation structure over the semiconductivesubstrate and laterally surrounding the semiconductive fin; asource/drain epitaxial structure over the semiconductive fin, whereinthe source/drain epitaxial structure has a rounded corner extendinglaterally and a top above the rounded corner; a first cap layerextending from the rounded corner of the source/drain epitaxialstructure to the top of the source/drain epitaxial structure; a secondcap layer covering the rounded corner and a bottom of the source/drainepitaxial structure, wherein the first and second cap layers are made ofdifferent materials; and a contact over the second cap layer, whereinthe first cap layer is a single piece of continuous material and incontact with the source/drain epitaxial structure, and the contact is incontact with a top surface of the first cap layer.
 2. The semiconductordevice of claim 1, wherein the first cap layer comprises silicon.
 3. Thesemiconductor device of claim 1, wherein the second cap layer comprisessilicon.
 4. The semiconductor device of claim 1, wherein the first caplayer and the second cap layer comprise different metals.
 5. Thesemiconductor device of claim 1, wherein the second cap layer is spacedapart from the top of the source/drain epitaxial structure.
 6. Thesemiconductor device of claim 1, wherein a bottommost surface of thesource/drain epitaxial structure is lower than a bottommost surface ofthe second cap layer.
 7. The semiconductor device of claim 1, whereinthe first cap layer comprises Ni, Co, Pt, W, Ru, or combinationsthereof.
 8. The semiconductor device of claim 1, wherein the second caplayer comprises Ti, Er, Y, Yb, Eu, Tb, Lu, Th, Sc, Hf, Zr, Tb, Ta, orcombinations thereof.
 9. A semiconductor device comprising: an isolationstructure over a substrate; a fin structure extending from the substrateand laterally surrounded by the isolation structure; a source/drainepitaxial structure over the fin structure, wherein the source/drainepitaxial structure comprises a round corner directly above theisolation structure and a bottom under the round corner; a first caplayer in contact with the round corner and the bottom of thesource/drain epitaxial structure, wherein a topmost surface of the firstcap layer is lower than a topmost surface of the source/drain epitaxialstructure; and a contact etch stop layer (CESL) covering the first caplayer.
 10. The semiconductor device of claim 9, wherein the first caplayer is a metal alloy layer.
 11. The semiconductor device of claim 9,further comprising a second cap layer over the source/drain epitaxialstructure and in contact with the first cap layer.
 12. The semiconductordevice of claim 11, wherein the second cap layer is a metal alloy layer.13. The semiconductor device of claim 11, wherein the CESL is in contactwith the second cap layer.
 14. The semiconductor device of claim 11,wherein the first and second cap layers both comprise Si.
 15. Asemiconductor device comprising: a substrate; a fin structure protrudingfrom the substrate; an isolation structure laterally surrounding the finstructure; a source/drain epitaxial structure over the fin structure; afirst cap layer in contact with a bottom of the source/drain epitaxialstructure and spaced apart from the isolation structure; a contact etchstop layer (CESL) in contact with the first cap layer and a top surfaceof the isolation structure; and a contact covering and electricallyconnected to the source/drain epitaxial structure, wherein a bottommostsurface of the contact is higher than a topmost surface of the first caplayer.
 16. The semiconductor device of claim 15, wherein the first caplayer comprises metals.
 17. The semiconductor device of claim 15,wherein a bottommost surface of the source/drain epitaxial structure islower than the top surface of the isolation structure.
 18. Thesemiconductor device of claim 15, wherein the source/drain epitaxialstructure has a rounded top.
 19. The semiconductor device of claim 15,wherein the source/drain epitaxial structure has a rounded cornerdirectly above the isolation structure, and the first cap layer is incontact with the rounded corner of the source/drain epitaxial structure.20. The semiconductor device of claim 15, further comprising a secondcap layer over and in contact with the source/drain epitaxial structureand the first cap layer, wherein the second cap layer comprises silicon.